1/2s=4s-21 I don't understand

= 4s - 21

First, simplify  to  / Your problem should look like: 
Second, multiply both sides by 2. / Your problem should look like: 
Third, subtract 8s from both sides. / Your problem should look like: 
Fourth, simplify s - 8s to -7s. / Your problem should look like: 
Fifth, divide both sides by -7. / Your problem should look like: 
Sixth, simplify  to  / Your problem should look like: 
Seventh, simplify  to 6. / Your problem should look like: 

 s = 6

D.

Bwould be the answer